The present invention relates generally to lithography, and particularly to optical photolithography glass for use in optical photolithography systems utilizing vacuum ultraviolet light (VUV) wavelengths below 193 nm, preferably below 175 nm, preferably below 164 nm, such as VUV projection lithography systems utilizing wavelengths in the 157 nm region.
The invention relates to VUV transmitting glass that is transmissive at wavelengths below 193 nm, in particular, a photomask silicon oxyfluoride glass suitable for use in the Vacuum Ultraviolet (VUV) 157 nm wavelength region.
U.S. Application Serial No. 60/271,136, filed Feb. 24, 2001, entitled Vacuum Ultra-Violet Transmitting Silicon Oxyfluoride Lithography Glass, to Lisa A. Moore and Charlene Smith, to U.S. application Ser. No. 09/397,572, filed Sep. 16, 1999 now U.S. Pat. No. 6,319,634, entitled Projection Lithography Photomasks And Method Of Making, of George Berkey, Lisa A. Moore and Michelle D. Pierson, and U.S. application Ser. No. 09/397,577 filed Sep. 16, 1999 now U.S. Pat. No. 6,265,115, entitled Projection Lithography Photomask Blanks, Preforms and Method of Making, of George Berkey, Lisa A. Moore and Charles C. Yu, are hereby incorporated by reference.
Refractive optics requires materials having high transmittance. For semiconductor applications where smaller and smaller features are desired at the 248 and 193 nm wavelengths, high purity fused silica has been show to exhibit the required minimum transmittance of 99%/cm or better.
Projection optical photolithography systems that utilize the vacuum ultraviolet wavelengths of light below 193 nm provide benefits in terms of achieving smaller feature dimensions. Such systems that utilize vacuum ultraviolet wavelengths in the 157 nm wavelength region have the potential of improving integrated circuits with smaller feature sizes. Current optical lithography systems used by the semiconductor industry in the manufacture of integrated circuits have progressed towards shorter wavelengths of light, such as the popular 248 nm and 193 nm wavelengths, but the commercial use and adoption of vacuum ultraviolet wavelengths below 193 nm, such as 157 nm has been hindered by the transmission nature of such vacuum ultraviolet wavelengths in the 157 nm region through optical materials. Such slow progression by the semiconductor industry of the use of VUV light below 175 nm such as 157 nm light has been also due to the lack of economically manufacturable photomask blanks from optically transmissive materials. For the benefit of vacuum ultraviolet photolithography in the 157 nm region such as the emission spectrum VUV window of a F2 excimer laser to be utilized in the manufacturing of integrated circuits there is a need for mask blanks that have beneficial optical properties including good transmission below 164 nm and at 157 nm and that can be manufactured economically.
The present invention overcomes problems in the prior art and provides a economical high quality improved photomask blanks and VUV transmitting lithography glass that can be used to improve the manufacturing of integrated circuits with vacuum ultraviolet wavelengths.
Use of high purity fused silica as optical elements in photolithography stems from the fact that high purity fused silica is transparent over a wide range of wavelengths, spanning from the infrared to deep ultraviolet regions. Furthermore, high purity fused silica exhibits excellent chemical durability and dimensional stability. These properties have made high purity fused silica highly suited for use as optical lenses as well as for photomask substrates in photolithography, but use has been limited to the KrF and ArF wavelength regions.
Photomask glass qualifications are comparatively different from other optical elements used in photolithography in that they characteristically have smaller thicknesses of as low as only a few millimeters through the optical path. As such, they must meet very strict requirements for dimensional stability (warping and shrinkage) in order to ensure the extreme accuracy required to form fine circuit patterns on the photomask plate and target. And as the demand for even smaller features continues to drive the lasing wavelength further down to the 157 nm region and lower, the choice of optical materials meeting the minimum required transmittance becomes severely limited for all optical elements, but even more so for photomask substrates due to the reasons stated above. Crystalline materials such as calcium fluoride, barium fluoride and magnesium fluoride for example, have been shown to exhibit transmittances which are suitable for 157 nm wavelength applications. Unfortunately, these materials tend to have certain drawbacks making them unsuitable for these applications, in addition to manufacturing/economic problems. For example, calcium fluoride exhibits unacceptably high thermal expansion properties for photomask applications in the 157 nm wavelength region. Magnesium fluoride on the other hand, exhibits acceptable expansion but is unsuitable because it is naturally birefringment.
Accordingly, it is an object of the present invention to disclose VUV transmitting silicon oxyfluoride glasses for use at VUV wavelengths below 193 nm, preferably in the F2 Excimer Laser 157 nm region, methods of making such glass, and methods for characterizing such silicon oxyfluoride glass.
In the present invention we disclose VUV transmitting silicon oxyfluoride lithography glass suitable for use as optical elements, for use as a lens or preferably for use as a photomask substrate at VUV wavelengths below 193 nm. In particular, the inventive silicon oxyfluoride glass exhibits certain properties tailored for applications in the photolithography VUV wavelength region around the 157 nm Excimer laser wavelengths and below 193 nm.
The object of the invention is achieved by use of a dry low hydroxy radical fluorine-doped SiO2 fused synthetic silicon oxyfluoride glass which exhibits very high transmittance in the vacuum ultraviolet (VUV) wavelength region while exhibiting excellent thermal and physical properties. By xe2x80x9cdryxe2x80x9d we mean having an OH content below 50 ppm by weight, preferably dehydrated below 10 ppm OH by weight, and most preferably below 1 ppm by weight.
In another aspect, the object of the invention is further achieved by ensuring that the silicon oxyfluoride glass is essentially free of chlorine.
In yet another aspect, the object of the invention is achieved by ensuring a low molecular hydrogen content in the glass. By this we mean that the molecular hydrogen (H2) content is below 1xc3x971017 molecules/cm3.
In a preferred embodiment of the invention, the VUV transmitting silicon oxyfluoride glass has a fluorine content in the range of 0.1 to 0.4 weight percent which inhibits laser exposure induced absorption and provides laser exposure durability with minimal transmission loss at 157.6 nm after prolonged exposure. The invention includes a below 193 nm VUV transmitting glass photomask substrate for photolithography at wavelengths of about 157 nm with the glass being a high purity silicon oxyfluoride glass with an OH content below 50 ppm by weight, hydrogen content below 1xc3x971017 molecules/cm3 and a fluorine content in the 0.1 to 0.4 weight percent range. The invention includes a process of making VUV transmitting glass silicon oxyfluoride glass that includes providing particles of SiO2, dehydrating the particles, and fluorine doping and consolidating the particles to form a dry, non-porous monolithic body of transparent fused silicon oxyfluoride glass with a fluorine content less than 0.5 weight percent. The invention includes a silicon oxyfluoride glass having essentially no OH groups, less than 5xc3x971016 molecules/cm3 of molecular hydrogen, and a fluorine content in the range of 0.1 to 0.4 weight %. The invention includes a silicon oxyfluoride lithography glass having an OH content less than 5 ppm by weight, a Cl content less than 5 ppm by weight, a H2 content less than 1xc3x971017 molecules/cm3, and a fluorine content of 0.1 to 0.4 weight % with a 157 nm internal transmission of at least 85%/cm. The invention includes a VUV pattern printing method with the steps of providing a below 164 nm radiation source for producing VUV photons, providing a silicon oxyfluoride glass having less than 5 ppm by weight OH, less than 5 ppm by weight Cl, a  less than 0.5 weight percent fluorine content, and 157 nm and 165 nm measured transmission of at least 75%/5 mm. The pattern printing method includes transmitting the VUV photons through the silicon oxyfluoride glass, forming a pattern with the VUV photons and projecting the pattern onto a VUV radiation sensitive printing pattern. The invention includes a VUV transmitting silicon oxyfluoride glass having a OH content less than 5 ppm by weight, a fluorine content of at least 0.1 weight %, the glass consisting essentially of Si, O, and F with an internal transmission in the wavelength range of 157 nm to 175 nm of at least 85%/cm and a 165 nm absorption less than 0.4 (absorption units/5 mm) after exposure to a 157 nm laser for 41.5 million pulses at 2 mJ/cm2-pulse.
Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate various embodiments of the invention, and together with the description serve to explain the principals and operation of the invention.